Methods for producing solid ceramic particles using a microwave firing process

ABSTRACT

Methods for producing solid, substantially round, spherical and sintered particles from a slurry of a raw material having an alumina content of greater than about 40 weight percent. The slurry is processed to prepare green pellets which are sintered in a furnace with microwave energy at a temperature of 1480 to 1520° C. to produce solid, substantially round, spherical and sintered particles having an average particle size greater than about 200 microns, a bulk density of greater than about 1.35 g/cm 3 , and an apparent specific gravity of greater than about 2.60.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of U.S. patentapplication Ser. No. 14/831,249, filed Aug. 20, 2015, which claimspriority to, and the benefit of the filing date of, U.S. PatentApplication No. 62/046,633, filed Sep. 5, 2014, the entire disclosuresof which are hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to oil and gas well proppants and, moreparticularly, to proppants exhibiting excellent crush resistance in abroad range of applications.

BACKGROUND

Oil and natural gas are produced from wells having porous and permeablesubterranean formations. The porosity of the formation permits theformation to store oil and gas, and the permeability of the formationpermits the oil or gas fluid to move through the formation. Permeabilityof the formation is essential to permit oil and gas to flow to alocation where it can be pumped from the well. Sometimes the oil or gasis held in a formation having insufficient permeability for economicrecovery of the oil and gas. In other cases, during operation of thewell, the permeability of the formation drops to the extent that furtherrecovery becomes uneconomical. In such cases, it is necessary tofracture the formation and prop the fracture in an open condition bymeans of a proppant material or propping agent. Such fracturing isusually accomplished by hydraulic pressure, and the proppant material orpropping agent is a particulate material, such as sand, glass beads orceramic particles, which are carried into the fracture by means of afluid.

Fracturing operations are more frequently being conducted at greaterdepths, which are under greater pressures. There is a need, therefore,for solid ceramic particles, and methods for making same, that haveincreased strength and crush resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a schematic illustration of a system for preparingsubstantially round and spherical particles from a slurry, according toseveral exemplary embodiments of the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. In other instances, well-knownstructures and techniques have not been shown in detail in order not toobscure the understanding of this description.

Described herein are methods for using microwave energy to fire andsinter proppants used in the hydraulic fracture stimulation of gas, oil,or geothermal reservoirs. Also described herein are themicrowave-sintered, substantially round and spherical particles andmethods for preparing such microwave-sintered, substantially round andspherical particles from a slurry of an alumina containing raw materialfor use as proppants. The term “substantially round and spherical” andrelated forms, as used herein, is defined to mean an average ratio ofminimum diameter to maximum diameter of about 0.8 or greater, or havingan average sphericity value of about 0.8 or greater compared to aKrumbein and Sloss chart.

According to several exemplary embodiments of the present invention, themicrowave-sintered, substantially round and spherical particles,referred to hereinafter as “microwave-sintered proppant” can be madefrom conventional pre-sintered proppants such as ceramic proppant, sand,plastic beads and glass beads. Such conventional proppants can bemanufactured up to the sintering step according to any suitable processincluding, but not limited to continuous spray atomization, sprayfluidization, spray drying, or compression. Suitable conventionalproppants and methods for their manufacture up to the sintering step aredisclosed in U.S. Pat. Nos. 4,068,718, 4,427,068, 4,440,866, 5,188,175,and 7,036,591, the entire disclosures of which are incorporated hereinby reference.

Ceramic proppants vary in properties such as apparent specific gravityby virtue of the starting raw material and the manufacturing process.The term “apparent specific gravity,” (ASG) as used herein, refers to anumber without units that is defined to be numerically equal to theweight in grams per cubic centimeter of volume, including void space,internal porosity or open porosity in determining the volume is theweight per unit volume (grams per cubic centimeter) of the particles.Low density proppants generally have an apparent specific gravity ofless than 3.0 g/cm³ and are typically made from kaolin clay and otheralumina, oxide, or silicate ceramics. Intermediate density proppantsgenerally have an apparent specific gravity of about 3.1 to 3.4 g/cm³and are typically made from bauxitic clay. High strength proppants aregenerally made from bauxitic clays with alumina and have an apparentspecific gravity above 3.4 g/cm³.

Ceramic proppant can also be manufactured in a manner that createsporosity in the proppant grain. A process to manufacture a suitableporous ceramic proppant is described in U.S. Pat. No. 7,036,591, theentire disclosure of which is incorporated herein by reference.

As described herein, the microwave sintered, substantially round andspherical particles are prepared from a slurry of an alumina-containingraw material. According to several exemplary embodiments, thesubstantially round and spherical particles can include alumina in anysuitable amounts. According to several exemplary embodiments, theproppant particulates include at least about 20 wt %, at least about 30wt %, at least about 35 wt %, or at least about 40 wt % alumina based onthe total weight of the particles. According to several exemplaryembodiments, the particulates include from about 20 wt % to about 99.9wt % alumina, from about 25 wt % to about 65 wt % alumina, from about 30wt % to about 55 wt % alumina, or from about 35 wt % to about 50 wt %alumina. In several exemplary embodiments, the particles include fromabout 10 wt % to about 90 wt % alumina, from about 25 wt % to about 75wt % alumina, from about 35 wt % to about 65 wt % alumina, from about 40wt % to about 55 wt % alumina, or from about 45 wt % to about 50 wt %alumina. In several exemplary embodiments, the sintered, substantiallyround and spherical particles include from about 41.5 wt % to about 49wt % alumina.

In several exemplary embodiments, the microwave sintered, substantiallyround and spherical particles have a bulk density of from about 1.30g/cm³, about 1.35 g/cm³, or about 1.42 g/cm³ to about 1.48 g/cm³, about1.55 g/cm³, or about 1.60 g/cm³. The term “bulk density”, (BD) as usedherein, refers to the weight per unit volume, including in the volumeconsidered, the void spaces between the particles. In several exemplaryembodiments, the particles have a bulk density of from about 1.40 g/cm³to about 1.50 g/cm³, from about 1.40 g/cm³ to about 1.45 g/cm³, or fromabout 1.45 g/cm³ to about 1.50 g/cm³.

In several exemplary embodiments, the microwave sintered, substantiallyround and spherical particles have a crush strength at 7,500 psi of fromabout 1%, about 1.5%, about 2.0%, or about 2.5% to about 3.0%, about3.5%, about 4.0%, or about 4.5%, long term fluid conductivity at 7,500psi of from about 1,475 millidarcy-feet (mD-ft), about 1,800 mD-ft,about 2,250 mD-ft, about 2,750 mD-ft, or about 3,500 mD-ft to about4,500 mD-ft, about 5,500 mD-ft, about 6,500 mD-ft, about 7,500 mD-ft, orabout 8,825 mD-ft and a long term permeability at 7,500 psi of fromabout 90 to about 480 darcies (D), about 150 D to about 475 D, about 250D to about 450 D, or about 375 D to about 425 D. In several exemplaryembodiments, the microwave sintered, substantially round and sphericalparticles have a crush strength at 7,500 psi of from about 1.5% to about3.2%, about 1.8% to about 2.9%, or about 2.1% to about 2.6%.

In several exemplary embodiments, the microwave sintered, substantiallyround and spherical particles have a long term fluid conductivity at10,000 psi of from about 2,000 mD-ft, about 2,250 mD-ft, or about 2,400mD-ft to about 2,500 mD-ft, about 2,650 mD-ft, or about 2,750 mD-ft anda long term permeability at 10,000 psi of from about 130 D to about 165D, about 140 D to about 160 D, or about 145 D to about 155 D. In severalexemplary embodiments, the microwave sintered, substantially round andspherical particles have a long term fluid conductivity at 12,000 psi offrom about 1,000 mD-ft, about 1,250 mD-ft, or about 1,400 mD-ft to about1,500 mD-ft, about 1,650 mD-ft, or about 1,750 mD-ft and a long termpermeability at 12,000 psi of from about 75 D to about 105 D, about 85 Dto about 100 D, or about 95 D to about 100 D.

According to several exemplary embodiments, the microwave sintered,substantially round and spherical particles have an apparent specificgravity (ASG) of less than 3.1 g/cm³, less than 3.0 g/cm³, less than 2.9g/cm³, less than 2.8 g/cm³, or less than 2.7 g/cm³. According to severalexemplary embodiments, the microwave sintered, substantially round andspherical particles have an apparent specific gravity of greater than2.0 g/cm³, greater than 2.4 g/cm³, greater than 2.6 g/cm³, or greaterthan 2.75 g/cm³. In several exemplary embodiments, the microwavesintered, substantially round and spherical particles have an apparentspecific gravity of from about 2.60 g/cm³ to about 2.80 g/cm³, fromabout 2.70 g/cm³ to about 2.80 g/cm³, from about 2.60 g/cm³ to about2.70 g/cm³, from about 2.75 g/cm³ to about 2.80 g/cm³, from about 2.70g/cm³ to about 2.75 g/cm³, from about 2.60 g/cm³ to about 2.65 g/cm³, orfrom about 2.65 g/cm³ to about 2.70 g/cm³.

According to several exemplary embodiments, the microwave sintered,substantially round and spherical particles have a size of from about 16to about 80 U.S. Mesh. According to several exemplary embodiments, themicrowave sintered, substantially round and spherical particles arescreened to provide product fractions designated as, for instance, 16/20mesh, 20/40 mesh, 30/50 mesh and 40/80 mesh. In conventional manner, the16/20 mesh sized product fraction passes through a 16 U.S. mesh (1190microns) screen, but is caught on a 20 U.S. mesh (841 microns) screen.The other sized product fractions are sized according to this conventionas well.

According to several exemplary embodiments, the microwave sintered,substantially round and spherical particles can have any suitable size.For example, the microwave sintered, substantially round and sphericalparticles can have a mesh size of at least about 6 mesh, at least about10 mesh, at least about 16 mesh, at least about 20 mesh, at least about25 mesh, at least about 30 mesh, at least about 35 mesh, or at leastabout 40 mesh. According to several exemplary embodiments, the microwavesintered, substantially round and spherical particles have a mesh sizefrom about 6 mesh, about 10 mesh, about 16 mesh, or about 20 mesh toabout 25 mesh, about 30 mesh, about 35 mesh, about 40 mesh, about 45mesh, about 50 mesh, about 70 mesh, or about 100 mesh. According toseveral exemplary embodiments, the microwave sintered, substantiallyround and spherical particles have a mesh size from about 4 mesh toabout 120 mesh, from about 10 mesh to about 60 mesh, from about 16 meshto about 20 mesh, from about 20 mesh to about 40 mesh, or from about 25mesh to about 35 mesh.

According to several exemplary embodiments described herein, themicrowave sintered, substantially round and spherical particles are madein a continuous process, while in several other exemplary embodiments,the particles are made in a batch process.

Referring now to the FIGURE, an exemplary system for implementing acontinuous process for preparing microwave sintered, substantially roundand spherical particles from a slurry is illustrated. Except for themicrowave sintering furnace, the exemplary system illustrated in theFIGURE can have a similar configuration and operation to that describedin U.S. Pat. No. 4,440,866, the entire disclosure of which isincorporated herein by reference. The operations performed by theexemplary system illustrated in the FIGURE can also be used to make theparticles according to a batch process.

In the system illustrated in the FIGURE, an alumina-containing rawmaterial having an alumina content of from about 10 wt % to about 90 wt%, from about 25 wt % to about 75 wt %, from about 35 wt % to about 65wt %, from about 40 wt % to about 55 wt %, or from about 45 wt % toabout 50 wt % (on a calcined basis) is passed through a shredder 105which slices and breaks apart the raw material into small chunks. Insome embodiments, when the raw material as mined, or as received,(referred to herein as “untreated” raw material) is of such consistencythat it can be processed as described herein without shredding, theshredder can be bypassed. Raw material fed through a shredder such as isillustrated in the FIGURE, is referred to as “treated” raw material.

In several exemplary embodiments, the shredder breaks apart and slicesthe alumina-containing raw material so as to yield pieces having adiameter of less than about 10 inches, less than about 7 inches, lessthan about 5 inches, less than about 3 inches, or less than about 1inch, although pieces having smaller and larger diameters can be furtherprocessed into a slurry as described herein. Shredders and numerousother devices for slicing, chopping or comminuting thealumina-containing raw material, as well as commercial sources for same,such as the Gleason Foundry Company, are well known to those of ordinaryskill in the art.

The treated or untreated alumina-containing raw material and water canbe fed to a blunger 110, which has a rotating blade that imparts a shearforce to and further reduces the particle size of the raw material toform a slurry. In a continuous process, the raw material and water arecontinuously fed to the blunger. Blungers and similar devices for makingslurries of such materials, as well as commercial sources for same arewell known to those of ordinary skill in the art.

A sufficient amount of water can be added to the blunger 110 to resultin a slurry having a solids content in the range of from about 10%,about 20%, about 40%, or about 50% to about 55%, about 60%, about 70%,or about 85% by weight. According to several exemplary embodiments, asufficient amount of water is added to the slurry such that the solidscontent of the slurry is from about 45% to about 55%, about 45% to about50%, or about 50% to about 65% by weight. According to several exemplaryembodiments, a sufficient amount of water is added to the slurry suchthat the solids content of the slurry is about 50% by weight. The wateradded to the blunger 110 can be fresh water or deionized water. In acontinuous process for preparing the slurry, the solids content of theslurry is periodically analyzed and the amount of water fed to theslurry adjusted to maintain the desired solids content. Methods foranalyzing the solids content of a slurry and adjusting a feed of waterare well known and understood by those of ordinary skill in the art.

According to several exemplary embodiments, a dispersant is added to theslurry in the blunger 110 to adjust the viscosity of the slurry to atarget range as discussed further below. In several exemplaryembodiments, the viscosity of the slurry in the blunger 110 is adjustedto the target range by the addition of a dispersant and a pH-adjustingreagent.

A dispersant can be added to the slurry prior to the addition of otheradditives. According to several exemplary embodiments, the compositionincludes a dispersant in an amount of from about 0.05%, about 0.10%,about 0.15%, or about 0.20% to about 0.25%, about 0.30%, about 0.35%, orabout 0.45% by weight based on the dry weight of the alumina-containingraw material.

Exemplary materials suitable for use as a dispersant in the compositionsand methods described herein include but are not limited to sodiumpolyacrylate, ammonium polyacrylate, ammonium polymethacrylate, tetrasodium pyrophosphate, tetra potassium pyrophosphate, polyphosphate,ammonium polyphosphate, ammonium citrate, ferric ammonium citrate, andpolyelectrolytes such as a composition of ammonium polymethacrylate andwater commercially available from a variety of sources, such as, KemiraChemicals under the trade name C-211, Phoenix Chemicals, Bulk ChemicalSystems under the trade name BCS 4020 and R.T. Vanderbilt Company, Inc.under the trade name DARVAN C. Generally, the dispersant can be anymaterial that will adjust the viscosity of the slurry to a targetviscosity such that the slurry can be subsequently processed through oneor more pressure nozzles of a fluidizer. In several exemplaryembodiments, the target viscosity is less than 150 centipoises (cps) (asdetermined on a Brookfield Viscometer with a #61 spindle). According toseveral exemplary embodiments, the target viscosity is less than 125cps, less than 100 cps, less than 80 cps, less than 70 cps, less than 60cps, less than 50 cps, less than 40 cps, less than 30 cps, or less than20 cps.

According to several exemplary embodiments in which a pH-adjustingreagent is used, a sufficient amount of a pH-adjusting reagent is addedto the slurry to adjust the pH of the slurry to a range of from about 8to about 11. In several exemplary embodiments, a sufficient amount ofthe pH-adjusting reagent is added to the slurry to adjust the pH toabout 9, about 9.5, about 10 or about 10.5. The pH of the slurry can beperiodically analyzed by a pH meter, and the amount of pH-adjustingreagent fed to the slurry adjusted to maintain a desired pH. Methods foranalyzing the pH of a slurry and adjusting the feed of the pH-adjustingreagent are within the ability of those of ordinary skill in the art.Exemplary materials suitable for use as a pH-adjusting reagent in thecompositions and methods described herein include but are not limited toammonia and sodium carbonate.

Generally, the target viscosity of the compositions is a viscosity thatcan be processed through a given type and size of pressure nozzle in afluidizer, without becoming clogged. Generally, the lower the viscosityof the slurry, the more easily it can be processed through a givenfluidizer. However, the addition of too much dispersant can cause theviscosity of the slurry to increase to a point that it cannot besatisfactorily processed through a given fluidizer. One of ordinaryskill in the art can determine the target viscosity for given fluidizertypes through routine experimentation.

The blunger 110 mixes the alumina-containing raw material, water,dispersant and pH-adjusting reagent until a slurry is formed. The lengthof time required to form a slurry is dependent on factors such as thesize of the blunger, the speed at which the blunger is operating, andthe amount of material in the blunger.

From the blunger 110, the slurry is fed to a tank 115, where the slurryis continuously stirred, and a binder is added in an amount of fromabout 0.01%, about 0.05%, about 0.1%, about 0.2%, about 0.5%, about1.0%, or about 2.0% to about 3.0%, about 5.0%, about 7.0%, about 10.0%,about 12.0%, about 15.0%, or about 20.0% by weight, based on the totaldry weight of the alumina-containing raw material. In several exemplaryembodiments, the binder is added in an amount of from about 0.2% toabout 3.0%, about 0.7% to about 2.5%, or about 1.5% to about 2.0% byweight based on the total dry weight of the alumina-containing rawmaterial. Suitable binders include but are not limited to polyvinylacetate, polyvinyl alcohol (PVA), methylcellulose, dextrin and molasses.In several exemplary embodiments, the binder is PVA having a molecularweight of from about 1,000 Mn, about 5,000 Mn, about 10,000 Mn, about20,000 Mn, or about 40,000 Mn to about 60,000 Mn, about 80,000 Mn, about100,000 Mn, about 120,000 Mn, or about 200,000 Mn. “Mn” represents thenumber average molecular weight which is the total weight of thepolymeric molecules in a sample, divided by the total number ofpolymeric molecules in that sample.

The tank 115 maintains the slurry created by the blunger 110. However,the tank 115 can stir the slurry with less agitation than the blunger,so as to mix the binder with the slurry without causing excessivefoaming of the slurry or increasing the viscosity of the slurry to anextent that would prevent the slurry from being fed through thepressurized nozzles of a fluidizer.

According to several exemplary embodiments, the binder can be added tothe slurry while in the blunger 110. According to such embodiments, theblunger 110 optionally has variable speeds, including a high speed toachieve the high intensity mixing for breaking down the raw materialinto a slurry form, and a low speed to mix the binder with the slurrywithout causing the above-mentioned excessive foaming or increase inviscosity.

Referring again to the tank 115 illustrated in the FIGURE, the slurry isstirred in the tank, after addition of the binder, for a time sufficientto thoroughly mix the binder with the slurry. In several exemplaryembodiments, the slurry is stirred in the tank 115 for up to about 30minutes following the addition of binder. In several exemplaryembodiments, the slurry is stirred in the tank 115 for at least about 30minutes. In several exemplary embodiments, the slurry is stirred in thetank for more than about 30 minutes after addition of the binder.

Tank 115 can also be a tank system comprised of one, two, three or moretanks. Any configuration or number of tanks that enables the thoroughmixing of the binder with the slurry is sufficient. In a continuousprocess, water, and one or more of dust, oversize particles, orundersize particles from a subsequent fluidizer or other apparatus canbe added to the slurry in the tank 115.

From the tank 115, the slurry is fed to a heat exchanger 120, whichheats the slurry to a temperature of from about 5° C., about 10° C.,about 15° C., about 25° C., about 35° C., or about 50° C. to about 65°C., about 75° C., about 90° C., about 95° C., about 99° C., or about105° C. From the heat exchanger 120, the slurry is fed to a pump system125, which feeds the slurry, under pressure, to a fluidizer 130.

A grinding mill(s) and/or a screening system(s) (not illustrated) can beinserted at one or more places in the system illustrated in the FIGUREprior to feeding the slurry to the fluidizer to assist in breaking anylarger-sized alumina-containing raw material down to a target sizesuitable for feeding to the fluidizer. In several exemplary embodiments,the target size is less than 230 mesh. In several exemplary embodiments,the target size is less than 325 mesh, less than 270 mesh, less than 200mesh or less than 170 mesh. The target size is influenced by the abilityof the type and/or size of the pressure nozzle in the subsequentfluidizer to atomize the slurry without becoming clogged.

If a grinding system is employed, it is charged with a grinding mediasuitable to assist in breaking the raw material down to a target sizesuitable for subsequent feeding through one or more pressure nozzles ofa fluidizer. If a screening system is employed, the screening system isdesigned to remove particles larger than the target size from theslurry. For example, the screening system can include one or morescreens, which are selected and positioned so as to screen the slurry toparticles that are smaller than the target size.

Referring again to the FIGURE, fluidizer 130 is of conventional design,such as described in, for example, U.S. Pat. No. 3,533,829 and U.K.Patent No. 1,401,303. Fluidizer 130 includes at least one atomizingnozzle 132 (three atomizing nozzles 132 being shown in the FIGURE),which is a pressure nozzle of conventional design. In other embodiments,one or more two-fluid nozzles are suitable. The design of such nozzlesis well known, for example from K. Masters: “Spray Drying Handbook”,John Wiley and Sons, New York (1979).

Fluidizer 130 further includes a particle bed 134, which is supported bya plate 136, such as a perforated, straight or directional plate. Hotair flows through the plate 136. The particle bed 134 comprises seedsfrom which green pellets of a target size can be grown. The term “greenpellets” and related forms, as used herein, refers to substantiallyround and spherical particles which have been formed from the slurry butare not sintered. When a perforated or straight plate is used, the seedsalso serve to obtain plug flow in the fluidizer. Plug flow is a termknown to those of ordinary skill in the art, and can generally bedescribed as a flow pattern where very little back mixing occurs. Theseed particles are smaller than the target size for green pellets madeaccording to the present methods. In several exemplary embodiments, theseed comprises from about 5% to about 20% of the total volume of a greenpellet formed therefrom. The slurry is sprayed, under pressure, throughthe atomizing nozzles 132, and the slurry spray coats the seeds to formgreen pellets that are substantially round and spherical.

External seeds can be placed on the perforated plate 136 beforeatomization of the slurry by the fluidizer begins. If external seeds areused, the seeds can be prepared in a slurry process similar to thatillustrated in the FIGURE, where the seeds are simply taken from thefluidizer at a target seed size. External seeds can also be prepared ina high intensity mixing process such as that described in U.S. Pat. No.4,879,181, the entire disclosure of which is hereby incorporated byreference.

According to several exemplary embodiments, external seeds are made fromeither a raw material having at least the same alumina content as theraw material used to make the slurry, or from a raw material having moreor less alumina than the raw material used to make the slurry. Inseveral exemplary embodiments, the slurry has an alumina content that isat least 10%, at least 20%, or at least 30% less than that of the seeds.In several exemplary embodiments, the external seeds have an aluminacontent less than that of the slurry, such as at least 10%, at least20%, or at least 30% less than that of the slurry.

Alternatively, seeds for the particle bed are formed by the atomizationof the slurry, thereby providing a method by which the slurry“self-germinates” with its own seed. According to several exemplaryembodiments, the slurry is fed through the fluidizer 130 in the absenceof a seeded particle bed 134. The slurry droplets exiting the nozzles132 solidify, but are small enough initially that they get carried outof the fluidizer 130 by air flow and caught as “dust” (fine particles)by a dust collector 145, which can, for instance, be an electrostaticprecipitator, a cyclone, a bag filter, a wet scrubber or a combinationthereof. The dust from the dust collector is then fed to the particlebed 134 through dust inlet 162, where it is sprayed with slurry exitingthe nozzles 132. The dust particles can be recycled a sufficient numberof times, until they have grown to a point where they are too large tobe carried out by the air flow and can serve as seeds. The dustparticles can also be recycled to another operation in the process, forexample, the tank 115.

Referring again to the FIGURE, hot air is introduced to the fluidizer130 by means of a fan and an air heater, which are schematicallyrepresented at 138. The velocity of the hot air passing through theparticle bed 134 can be from about 0.1 meters/second, about 0.5meters/second, or about 0.9 meters/second to about 1.2 meters/second,about 1.5 meters/second, or about 2.0 meters/second, and the depth ofthe particle bed 134 can be from about 1 centimeter, about 2centimeters, about 5 centimeters, about 10 centimeters, or about 20centimeters to about 30 centimeters, about 40 centimeters, about 60centimeters, about 80 centimeters, or about 100 centimeters. Thetemperature of the hot air when introduced to the fluidizer 130 can befrom about 100° C., about 150° C., about 200° C., or about 250° C. toabout 300° C., about 400° C., about 500° C., about 600° C., about 650°C., or about 1,000° C. The temperature of the hot air as it exits fromthe fluidizer 130 can be less than about 250° C., less than about 200°C., or less than about 150° C., and in several exemplary embodiments isless than about 100° C.

The distance between the atomizing nozzles 132 and the plate 136 isoptimized to avoid the formation of dust which occurs when the nozzles132 are too far away from the plate 126 and the formation of irregular,coarse particles which occurs when the nozzles 132 are too close to theplate 136. The position of the nozzles 132 with respect to the plate 136is adjusted on the basis of an analysis of powder sampled from thefluidizer 130.

The green pellets formed by the fluidizer accumulate in the particle bed134. In a continuous process, the green pellets formed by the fluidizer130 are withdrawn through an outlet 140 in response to the level ofproduct in the particle bed 134 in the fluidizer 130, so as to maintaina given depth in the particle bed. A rotary valve 150 conducts greenpellets withdrawn from the fluidizer 130 to an elevator 155, which feedsthe green pellets to a screening system 160, where the green pellets areseparated into one or more fractions, for example, an oversize fraction,a product fraction, and an undersize fraction.

The oversize fraction exiting the screening unit 160 includes thosegreen pellets that are larger than the desired product size. In acontinuous process, the oversize green pellets can be recycled to tank115, where at least some of the oversize green pellets can be brokendown and blended with slurry in the tank. Alternatively, oversize greenpellets can be broken down and recycled to the particle bed 134 in thefluidizer 130. The undersize fraction exiting the screening system 160includes those green pellets that are smaller than the desired productsize. In a continuous process, these green pellets can be recycled tothe fluidizer 130, where they can be fed through an inlet 162 as seedsor as a secondary feed to the fluidizer 130.

The product fraction exiting the screening system 160 includes thosegreen pellets having the desired product size. The size limits for theproduct fractions exiting the screening system 160 are fixed with dueregard to the fact that in the subsequent sintering process, substantialshrinkage of the green pellets can occur depending upon the startingmaterials from which the green pellets are made. The green pelletsexiting the screening system 160 can be sent to a pre-sintering device165, for example, a calciner, where the green pellets are dried orcalcined prior to sintering. In several exemplary embodiments, the greenpellets are dried in the pre-sintering device 165 to a moisture contentof less than about 25% by weight, less than about 18% by weight, lessthan about 15% by weight, less than about 12% by weight, less than about10% by weight, less than about 5% by weight, or less than about 1% byweight. The pre-sintering device 165 can be or include any suitabledevice for removing moisture from the green pellets. In an exemplaryembodiment, the pre-sintering device 165 can include a calciner. Thecalciner can be or include one or more direct and/or indirect firedrotary kilns.

In several exemplary embodiments, after drying and/or calcining, thegreen pellets are fed to a microwave furnace 170, in which the greenpellets are sintered for a period of time and at a temperaturesufficient to enable recovery of microwave sintered, substantially roundand spherical particles having one or more of a desired apparentspecific gravity, bulk density, and crush strength. According to severalexemplary embodiments, the microwave furnace 170 is a rotary furnacehaving a rotating section (not shown) and non-rotating sections (notshown) disposed at either end of the rotating section. In an exemplaryembodiment, the microwave furnace 170 can include a rotating sectionhaving a first end and a second end. A first non-rotating section can becoupled to or in fluid communication with the first end of the rotatingsection and a second non-rotating section can be coupled to or in fluidcommunication with the second end of the rotating section. In anexemplary embodiment, the rotating section can include an interior orcavity. At least a portion of the cavity of the rotating section can belined with a refractory lining. The refractory lining can includealumina oxide, silica oxide, magnesia oxide, or any suitable combinationor mixture thereof.

The microwave furnace 170 can include any suitable number andarrangement of microwave generators. The microwave furnace can includefrom 1, 2, 3, or 4 to 6, 8, 10, or 20 microwave generators. Themicrowave generators can be coupled to the non-rotating sections of themicrowave furnace 170 at any suitable location. For example, 1, 2, 3, 4,5, or 6 or more microwave generators can be disposed about thecircumference of the first and/or second non-rotating sections to directmicrowave energy into the cavity of the rotating section. The microwavegenerators disposed about the circumference of the non-rotatingsection(s) can be axially aligned or axially offset with respect to oneanother. The 1, 2, 3, 4, 5, or 6 or more microwave generators can alsobe spaced apart and disposed along the length of the non-rotatingsection(s) to direct microwave energy into the cavity of the rotatingsection. One or more of the microwave generators can generate atemperature of about 200° C., about 350° C., about 500° C., about 650°C., or about 800° C. to about 1,000° C., about 1,200° C., about 1,350°C., about 1,400° C., or about 1,550° C. or more in the cavity of therotating section. In an exemplary embodiment, the maximum temperaturethat can be generated in the cavity by the microwave generators is about1,600° C., about 1,620° C., about 1,650° C., or about 1,700° C. The oneor more microwave generators can achieve a heating rate of from about 1°C./min, about 2° C./min, about 5° C./min, about 7° C./min, or about 9°C./min to about 11° C./min, about 13° C./min, about 15° C./min, about20° C./min, or about 25° C./min in the cavity of the rotating section.

Alternatively, the pre-sintering device 165 can be eliminated if themicrowave furnace 170 can provide sufficient calcining and/or dryingconditions (i.e., drying times and temperatures that dry the greenpellets to a target moisture content prior to sintering), followed bysufficient sintering conditions. The green pellets exiting the screeningsystem 160 can be sent, either directly or indirectly, to the microwavefurnace 170, where the green pellets are dried, calcined, and/orsintered. In several exemplary embodiments, the green pellets introducedto the microwave furnace have a moisture content of at least about 25%by weight, at least about 18% by weight, at least about 15% by weight,at least about 12% by weight, at least about 10% by weight, at leastabout 5% by weight, or at least about 1% by weight.

The specific time and temperature to be employed for sintering isdependent on the starting ingredients and the desired density for thesintered particles. In several exemplary embodiments, the microwavefurnace 170 is a continuous microwave furnace, operating at a peaktemperature of from about 1,350° C., about 1,420° C., or about 1,480° C.to about 1,520° C., about 1,580° C., or about 1,620° C. and the greenpellets are sintered at the peak temperature of the continuous microwavefurnace for a cycle time of from about 5, about 10, about 15 to about20, about 30, about 40, about 45, or about 60 minutes or more. Inseveral exemplary embodiments, the continuous microwave furnace has apre-heat zone, a sintering zone and a cooling zone. According to suchseveral exemplary embodiments, the pellets have a residence time in thepre-heat, sintering and cooling zones of the microwave furnace of about30 minutes to about 120 minutes, about 50 minutes to about 200 minutes,and about 60 minutes to about 240 minutes, respectively.

According to several exemplary embodiments, the microwave-sintered,substantially round and spherical particles, or microwave-sinteredproppant described above, is withdrawn from the microwave furnace 170.After the particles exit the microwave furnace 170, they can be furtherscreened for size, and tested for quality control purposes.

The following examples are illustrative of the methods and particlesdiscussed above.

EXAMPLES

In the following examples, green pellets for forming various sizes ofconventional low density and medium density ceramic proppants wereprepared for sintering in a continuous microwave furnace. The greenpellets were sized with shrinkage in mind for ultimate production of16/20 mesh, 20/40 mesh, 30/50 mesh and 40/80 mesh-sized proppantsamples. The green pellets were those that are used to make productsmarketed under the trade names CARBOLite (CL) 16/20, CARBOLite (CL)20/40, CARBOLite (CL) 30/50 and HydroProp 40/80 which are commerciallyavailable from CARBO Ceramics, Inc. of Houston, Tex.

The green pellets of various sizes were processed in a continuousmicrowave processing system having a pusher-type furnace usingrectangular crucibles/boat having an approximate capacity of about 400grams of proppant material. A total of 28 such boats were in the tunnelof the continuous microwave processing system at any given time duringcontinuous pusher operation. The pusher system had a lateral speed of 15mm to 600 mm per hour, a microwave power output of 0.9 to 12 KW and atemperature range of 450° C. to 1,500° C.

The green particles were sintered in the tunnel of the continuousmicrowave processing system to produce substantially round and sphericalsolid ceramic particles. Sintering was achieved using a heating rate of10° C./min to a peak temperature of about 1,520° C. with various timesat peak temperature. In the results discussed below, the bulk density,apparent specific gravity and crush strength of the sintered solidceramic particles were determined using API Recommended Practices RP60for testing proppants.

In the continuous microwave processing system, the temperature wasmonitored with thermocouples at 4 points in the system as follows: T1 inthe pre-heat zone, T2 and T3 in the sinter-heat zone and T4 in thecooling zone. T1, T2 and T3 provide feedback for microwave powerregulation and temperature control in the pre-heat and sinter-heatzones, while T4 monitors the temperature in the cooling zone.

The sample boats in the continuous microwave furnace had lids on them tofacilitate uniform heating of their contents. The lids did not haveholes, and the temperature within a closed lid sample boat was measuredto be approximately 100° C. higher in the closed boat with a lid surfacetemperature around 1,400° C. The increase in temperature caused by thelids enabled the green pellet samples to be sintered at a peaktemperature of 1,520° C. even though the maximum output temperature ofthe microwave furnace was 1,500° C.

Based on the design and microwave power input to the different regionsof the microwave furnace, the maximum temperature possible at T1 in thepreheat zone was up to about 700° C. and the maximum temperature for T2and T3 in the sinter-heat zone was up to about 1,500° C. The fastestcycle time (frequency of boat input) possible was 5 min.

Five different sets of trials were carried out as set forth below inExamples 1-5.

Example 1 5 Sample Boats of CL16/20, Cycle Times 10 Min. & 5 Min

According to Example 1, 5 boats with proppant samples were processed inthe continuous microwave processing system. The sample boats were fedinto the continuous microwave processing system with a cycle time of 10minutes except for the fifth sample boat for which the cycle time was 5minutes.

The 5 samples were assigned ref nos. 1A-1E and the results are shownbelow in Table 1.

TABLE 1 Approx. Cycle Meas. Target Crush Target Sample Kiln Peak Time BDBD (%) at Crush(%) ID Material Temp (° C.) (min) (g/cm³) (g/cm³) 7500psi at 7500 psi 1A CL16/20 1446 10 1.582 1.57 12.5 14 1B CL16/20 1443 101.564 1.57 12.7 14 1C CL16/20 1440 10 1.555 1.57 12.5 14 1D CL16/20 143710 1.538 1.57 13.3 14 1E CL16/20 1430 5 1.527 1.57 13.4 14

As shown in TABLE 1, the measured bulk density (BD) of the microwavesintered samples dropped as the peak temperature of the microwavefurnace dropped.

Also as shown in TABLE 1, the crush percentage of the microwave sinteredsamples increased as the peak temperature of the microwave furnacedropped. Nevertheless, the crush percentage of the microwave sinteredsamples was lower in each instance than the target crush percentage ofcomparable conventional rotary kiln sintered proppant even when themeasured bulk density of the microwave sintered samples dropped belowthe target bulk density values.

Example 2 3 Sample Boats, 1 each of CL20/40, CL30/50 & HP40/80, CycleTime 15 minutes

According to Example 2, 3 sample boats—one each of CL20/40, CL30/50 &HP40/80—were processed in the continuous microwave processing system.The sample boats were fed into the continuous microwave processingsystem with a cycle time of 15 minutes.

The samples were assigned ref. nos. 2A, 2B & 2C, respectively, and theresults are shown below in Table 2.

TABLE 2 API Target Peak Cycle Meas Target Crush Crush Sample Temp TimeBD BD Meas. Target at at ID Material (° C.) (min) (g/cm³) (g/cm³) ASGASG 7500 psi 7500 psi 2A CL20/40 1414 15 1.56 1.56 2.77 2.72 3.1 4.1 2BCL30/50 1414 15 1.54 1.54 2.78 2.72 1.5 1.9 2C HP40/80 1419 15 1.38 1.432.68 2.55 2.5 2.8

In each instance, the measured bulk density of the microwave processedsamples was comparable or slightly below the target values. The ASG andcrush percentage values, however, for the samples prepared according toExample 2, were better in each instance in comparison to the targetvalues from conventional processing.

Example 3 10 Sample Boats, 5 Boats Each of CL20/40 & CL30/50, Cycle Time15 Min

According to Example 3, 10 sample boats, five each of CL20/40 & CL30/50were processed in the continuous microwave processing system. The sampleboats were fed into the continuous microwave processing system with acycle time of 15 minutes.

The different sample groups were assigned ref nos. 3(A-E) & 3(F-J) andthe results are shown below in Table 3.

TABLE 3 API Peak Cycle Meas Target Crush Target Sample Temp Time BD BDMeas. Target at Crush ID Material (° C.) (min) (g/cm³) (g/cm³) ASG ASG7500 psi value 3A CL20/40 1426 15 1.57 1.56 2.78 2.72 2.8 4.1 3B CL20/401426 15 1.57 1.56 2.78 2.72 4.1 3C CL20/40 1429 15 1.57 1.56 2.78 2.724.1 3D CL20/40 1429 15 1.57 1.56 2.77 2.72 4.1 3E CL20/40 1421 15 1.581.56 2.78 2.72 2.0 4.1 3F CL30/50 1406 15 1.54 1.54 2.77 2.72 1.3 1.9 3GCL30/50 1412 15 1.54 1.54 2.77 2.72 1.9 3H CL30/50 1412 15 1.54 1.542.78 2.72 1.9 3I CL30/50 1407 15 1.54 1.54 2.77 2.72 1.9 3J CL30/50 140615 1.54 1.54 2.77 2.72 1.5 1.9

As shown in Table 3, the measured bulk densities of the CL20/40 andCL30/50 microwave processed samples were comparable to or better thanthe target values from conventional processing.

The ASG and crush values of the CL20/40 and CL30/50 microwave processedsamples with a cycle time of 15 minutes were significantly better incomparison to the target values from conventional processing.

Example 4 5 Sample Boats of HP40/80, Cycle Time 15 Minutes

According to Example 4, 5 sample boats of HP40/80 were processed in thecontinuous microwave processing system. The sample boats were fed intothe continuous microwave processing system with a cycle time of 15minutes.

The five samples were assigned ref nos. 4(A-E) and the results are shownbelow in Table 4.

TABLE 4 API Peak Cycle Meas Target Crush Target Sample Temp Time BD BDMeas. Target at Crush ID Material (° C.) (min) (g/cm³) (g/cm³) ASG ASG7500 psi value 4A HP40/80 ~1440 15 1.39 1.43 2.71 2.55 2.4 2.8 4BHP40/80 ~1440 15 1.39 1.43 2.68 2.55 2.8 4C HP40/80 ~1440 15 1.40 1.432.71 2.55 2.8 4D HP40/80 ~1440 15 1.39 1.43 2.71 2.55 2.8 4E HP40/80~1440 15 1.39 1.43 2.70 2.55 2.1 2.8

The measured bulk density of the HP40/80 microwave processed samples wasbelow target values from conventional processing.

The ASG values of the HP40/80 microwave processed samples with a cycletime of 15 minutes were slightly higher compared to the target valuesfrom conventional processing. The crush values of the HP40/80 microwaveprocessed samples with a cycle time of 15 minutes were lower compared tothe target values from conventional processing.

Example 5 5 Sample Boats of CL20/40, Cycle Time 20 Minutes

According to Example 5, a set of 5 sample boats of CL20/40 wereprocessed in the continuous microwave processing system. The sampleboats were fed into the continuous microwave processing system with acycle time of 20 minutes.

The five samples were assigned ref nos. 5(A-E) and the results are shownbelow in Table 5.

TABLE 5 API Peak Cycle Meas Target Crush Target Sample Temp Time BD BDMeas. Target at Crush ID Material (° C.) (min) (g/cm³) (g/cm³) ASG ASG7500 psi value 5A CL20/40 1418 20 1.57 1.56 2.78 2.72 3.0 4.1 5B CL20/401423 20 1.57 1.56 2.78 2.72 4.1 5C CL20/40 1429 20 1.57 1.56 2.78 2.724.1 5D CL20/40 1429 20 1.27 1.56 2.77 2.72 4.1 5E CL20/40 1426 20 1.571.56 2.78 2.72 4.4 4.1

The measured bulk density of the CL20/40 microwave processed sampleswith a cycle time of 20 minutes was comparable to target values fromconventional processing.

The ASG of the CL20/40 microwave processed samples with a cycle time of20 minutes was comparable to similar samples from Example 3.

The crush value of the CL20/40 microwave processed samples with a cycletime of 20 minutes was not as good as that obtained for similar samplesfrom Example 3, which samples were microwave processed with a cycle timeof 15 minutes.

It was observed from the results of Examples 1-5, that the measured BD(bulk density) and crush percent values tended to deteriorate with adrop of microwave furnace/kiln peak temperature. However, the measuredcrush percent values of microwave processed samples were better than thetarget values from conventional processing even when the measured bulkdensity values of microwave processed samples dropped below the targetbulk density values from conventional processing.

The effect of increased cycle time on microwave processed sampleproperties was investigated. The crush value of the CL20/40 microwaveprocessed samples with a cycle time of 20 minutes were higher than thecrush values obtained for similar samples that were microwave processedwith a cycle time of 15 minutes.

Example 6

According to Example 6, two samples of proppant sample 3D from Example 3above, which were CL20/40 samples that were processed with a cycle timeof 15 minutes and two samples of a conventional CL20/40 proppant thatwere sintered in a gas-fired rotary furnace, were tested in terms oflong term conductivity and long term permeability.

The 4 samples were assigned ref nos. 6A-6D and the results are shownbelow in Table 6.

TABLE 6 Sample Long Term Conductivity (mD-ft) Long Term Permeability (D)Sample mass 2k 4k 6k 8k 10k 12k 2k 4k 6k 8k 10k 12k ID Description (g)psi psi psi psi psi psi psi psi psi psi psi psi 6A Conventional 63.0 g8615 7920 6052 4026 2706 1683 481 439 339 233 161 104 20/40 CL 6B Sample3D 63.0 g 8659 7696 5824 3847 2584 1575 470 424 326 222 154 98 fromExample 3 6C Conventional 63.0 g 8680 8066 6193 3941 2591 1570 482 446348 230 155 98 20/40 CL 6D Sample 3D 63.0 g 8822 7601 5760 3892 24981479 480 414 321 223 148 91 from Example 3

The long term conductivity for samples 3D from Example 3 ranged from1,479 to 8,822 millidarcy-feet (mD-ft) compared to 1,570 to 8,680 mD-ftfor the conventional CL20/40 samples. Also, the long term permeabilityfor samples 3D from Example 3 ranged from 91 to 480 darcies (D) comparedto 98 to 482 D for the conventional CL20/40 samples. The long termconductivity and the long term permeability were each measured underclosure stresses of 2,000 psi, 4,000 psi, 6,000 psi, 8,000 psi, 10,000psi, and 12,000 psi. These results demonstrated that the results ofsintering in a conventional gas-fired rotary kiln in terms of long termconductivity and long term permeability can be approached by sinteringin a microwave furnace.

Exemplary embodiments of the present disclosure further relate to anyone or more of the following paragraphs:

1. A method for forming sintered substantially round and sphericalpellets, comprising: forming substantially round and spherical greenpellets from raw materials comprising water and kaolin clay; andsintering the green pellets in a microwave furnace at a temperature offrom about 1480° C. to about 1520° C. for a time at peak temperature offrom about 20 to about 45 minutes, to form a proppant having a bulkdensity of from about 1.35 to about 1.60 g/cm³ and a crush percent at7500 psi of from about 1.5 to about 3.5; wherein the proppant has a bulkdensity and a crush percent at 7500 psi that is less than that ofproppant made from the green pellets and sintered in a gas-fired rotarykiln.

2. The method according to paragraph 1, wherein the proppant has anapparent specific gravity of about 2.6 to about 2.8.

3. The method according to paragraphs 1 or 2, wherein the proppant has along term conductivity of about 1475 mD-ft to about 8825 mD-ft and along term permeability of from about 90 D to about 480 D.

4. The method according to any one of paragraphs 1 to 3, wherein thegreen pellets have a size of from 16 to 80 U.S. mesh.

5. The method according to any one of paragraphs 1 to 4, wherein thegreen pellets are sintered in the microwave furnace for about 50 toabout 200 minutes.

6. The method according to any one of paragraphs 1 to 5, wherein thesubstantially round and spherical green pellets are prepared from aslurry having a solids content of from about 40 to about 60% by weightand having an alumina content in a range of from about 40 to about 55%by weight.

7. A method for producing sintered particles comprising: preparing aslurry having a solids content of from about 40 to about 60% by weight,and comprising water and a raw material having an alumina content in arange of from about 40 to about 55% by weight; atomizing the slurry intodroplets; coating seeds comprising alumina with the droplets to formgreen particles; and sintering the green particles in a microwavefurnace at a temperature of from about 1480° C. to about 1520° C. for atime at peak temperature of from about 20 to about 45 minutes, to form aproppant having a bulk density of from about 1.35 to about 1.60 g/cm³and a crush percent at 7500 psi of from about 1.5 to about 3.5; whereinthe proppant has a bulk density and a crush percent at 7500 psi that isless than that of proppant made from the green particles and sintered ina gas-fired rotary kiln.

8. The method according to paragraph 7, wherein the green particles aresintered for about 50 to about 200 minutes.

9. The method according to paragraphs 7 or 8, further comprising addingto the slurry at least one of a pH-adjusting reagent, a dispersant and adefoamer before atomizing the slurry.

10. The method according to any one of paragraphs 7 to 9, whereinatomizing the slurry comprises feeding the slurry to a fluidizeroperable to atomize the slurry into droplets; and wherein the seeds arepositioned in a particle bed in the fluidizer.

11. The method according to any one of paragraphs 7 to 10, furthercomprising drying the green particles before sintering.

12. A system for producing sintered particles comprising: a source ofraw material having an alumina content in a range of from about 40 toabout 55% by weight; a blunger operable to receive a feed of the rawmaterial, and to prepare a slurry from the raw material by mixing theraw material and water; a heat exchanger operable to heat the slurry toa temperature of from about 25° C. to about 90° C.; a fluidizercomprising at least one nozzle and a particle bed, wherein the fluidizeris operable to receive a pressurized feed of the slurry and pump theslurry under pressure through the at least one nozzle as a spray, andwherein the particle bed is populated with seeds, which seeds are coatedwith the slurry spray to form particles; and a microwave furnaceoperable to receive and sinter at least a portion of the particlesformed by the fluidizer, which sintering is performed at a temperatureof from about 1480° C. to about 1520° C. for a time at peak temperatureof from about 20 to about 45 minutes, to form sintered particles havinga bulk density of from about 1.35 to about 1.60 g/cm³ and a crushpercent at 7500 psi of from about 1.5 to about 3.5.

13. The system according to paragraph 12, further comprising at leastone of: a tank operable to receive a feed of the slurry from theblunger, and to mix a binder with the slurry; a grinding mill operableto grind particles in the slurry to a target size before the slurry isfed to the fluidizer; a pump system operable to receive a feed of theslurry from the heat exchanger and to provide a pressurized feed of theslurry to the fluidizer; and a drier operable to dry the particlesbefore the particles are sintered.

14. The system according to paragraphs 12 or 13, further comprising atleast one of: a first screening unit operable to screen the slurry to atarget size before the slurry is fed to the fluidizer; a secondscreening unit operable to receive the particles from the fluidizer andscreen the particles for size prior to sintering; and a third screeningunit operable to receive the sintered particles from the sinteringdevice and screen the sintered particles for size.

15. A substantially round and spherical sintered particle comprising: aseed comprising alumina; and a coating comprising alumina, wherein thecoating comprises at least about 80% of the total volume of thesubstantially round and spherical sintered particle; wherein thesintered particle has: a bulk density in a range of from about 1.35g/cm³ to about 1.60 g/cm³; a crush strength at 7500 psi of from about1.5 percent to about 3.5 percent; a short term conductivity of fromabout 1475 mD-ft to about 8825 mD-ft.; and a long term permeability offrom about 90 D to about 480 D.

16. A method of fracturing a subterranean formation comprising:injecting a hydraulic fluid into the subterranean formation at a rateand pressure sufficient to open a fracture therein; and injecting afluid containing substantially round and spherical sintered particlesinto the fracture, wherein the substantially round and sphericalsintered particles: have an alumina content of about 40% by weight toabout 55% by weight, and a bulk density of about 1.35 g/cm³ to about1.60 g/cm³; and were formed by: preparing a slurry having a solidscontent of about 40% to about 60% by weight, and comprising water and araw material having an alumina content in a range of from about 40% byweight to about 55% by weight; atomizing the slurry into droplets;coating seeds comprising alumina with the droplets to form greenparticles; and sintering in a microwave furnace at least a portion ofthe green particles at a temperature of from about 1480° C. to about1520° C. for a time at peak temperature of from about 20 to about 45minutes.

Certain embodiments and features have been described using a set ofnumerical upper limits and a set of numerical lower limits. It should beappreciated that ranges from any lower limit to any upper limit arecontemplated unless otherwise indicated. Certain lower limits, upperlimits and ranges appear in one or more claims below. All numericalvalues are “about” or “approximately” the indicated value, and take intoaccount numerical error and variations that would be expected by aperson having ordinary skill in the art.

Various terms have been defined above. To the extent a term used in aclaim is not defined above, it should be given the broadest definitionpersons in the pertinent art have given that term as reflected in atleast one printed publication or issued patent. Furthermore, allpatents, test procedures, and other documents cited in this applicationare fully incorporated by reference to the extent such disclosure is notinconsistent with this application and for all jurisdictions in whichsuch incorporation is permitted.

The substantially round and spherical solid ceramic particles that areproduced according to the methods described herein are suitable for avariety of uses, including but not limited to use as a proppant in oilor gas wells, and as a foundry media. Other embodiments of the currentinvention will be apparent to those skilled in the art from aconsideration of this specification or practice of the inventiondisclosed herein. However, the foregoing specification is consideredmerely exemplary of embodiments of the invention with the true scope andspirit of the invention being indicated by the following claims.

What is claimed is:
 1. A method for forming sintered substantially roundand spherical pellets, comprising: forming substantially round andspherical green pellets from raw materials comprising water and kaolinclay; and sintering the green pellets in a microwave furnace at atemperature of from about 1350° C. to about 1620° C. for a time at peaktemperature of from about 20 to about 200 minutes, to form a proppanthaving a bulk density of from about 1.3 to about 1.6 g/cm³ and a crushpercent at 7500 psi of from about 1.5 to about 3.5.
 2. The method ofclaim 1, wherein the proppant has an apparent specific gravity ofgreater than
 2. 3. The method of claim 1, wherein the proppant has anapparent specific gravity of less than 3.1.
 4. The method of claim 1,wherein the proppant has an apparent specific gravity of about 2.6 toabout 2.8.
 5. The method of claim 1, wherein the green pellets aresintered in the microwave furnace at a peak temperature of from about1480° C. to about 1520° C. for about 20 to about 45 minutes.
 6. Themethod of claim 1, wherein the substantially round and spherical greenpellets are prepared from a slurry having a solids content of from about40 to about 60% by weight and having an alumina content in a range offrom about 40 to about 55% by weight.
 7. A method for producing sinteredparticles comprising: forming green particles having a moisture contentof at least about 5% by weight from a slurry having a solids content offrom about 40 to about 60% by weight, and comprising water and a rawmaterial having an alumina content in a range of from about 40 to about55% by weight; drying the green particles in a microwave furnace toproduce dried green particles having a moisture content of less than 5%by weight; and sintering the dried green particles at a temperature offrom about 1480° C. to about 1520° C. for a time at peak temperature offrom about 20 to about 45 minutes, to form sintered particles having abulk density of from about 1.35 to about 1.60 g/cm³ and a crush percentat 7500 psi of from about 1.5 to about 3.5.
 8. The method of claim 7,wherein the dried green particles are sintered for about 50 to about 200minutes.
 9. The method of claim 7, further comprising calcining thedried green pellets in the microwave furnace prior to sintering.
 10. Themethod of claim 7, wherein the sintering is performed in the microwavefurnace.
 11. The method of claim 7, wherein the sintered particles havean apparent specific gravity of about 2.6 to about 2.8.